1. Field of the Invention
This invention relates to a process for producing improved quality coke without contamination by recycle materials. In one embodiment, a high-quality premium (needle) coke is produced by using an aromatic concentrate as the feed and a non-coking feed supplement in lieu of recycle material from a fractionator.
More particularly, this invention concerns a process in which premium coker feed is combined with a non-coking feed supplement, fed directly to a furnace, heated and charged to a coke drum. Vapors from the premium coking process can be returned to a fractionator which is concurrently utilized in a regular grade coking operation. The premium coke thus formed is of a higher quality than coke made from conventional fractionator bottoms.
2. Discussion of the Prior Art
Production of premium petroleum coke of the quality needed to produce large graphite electrodes for use in electric arc furnaces remains more of an art than a predictable process, and improvements are constantly being sought. Regular (non-premium) petroleum coke is amorphous with a dark, spongy appearance and breaks into lumps of irregular shape. Premium coke is distinguished from regular or ordinary coke by its crystalline appearance and tendency to fracture into long, needle-like particles when crushed. After green coke preparation, coke calcination, electrode formation, and baking and graphitization, high-quality premium or needle coke has a low coefficient of thermal expansion (CTE), commonly below 7.0.times.10.sup.-7 /.degree.C. Characteristics of premium coke are discussed in several references and patents including U.S. Pat. No. 4,130,475 to Cameron et al.
Delayed coking is a process in which a petroleum residue (or similar hydrocarbon fraction) is heated to a condition of thermal instability. Cokable materials are withdrawn from the bottom of a fractionator and pumped through a furnace at a high velocity to prevent coke formation on the inner walls of the furnace tubes. The high velocity is maintained in the furnace outlet piping and in the transfer line to the coke drums for the same reason. The coke drums are reservoirs that allow the combination of temperature and time to complete the thermal cracking initiated in the furnace. Vapors from the process exit the top of the coke drum and are usually returned to the coker fractionator. Delayed coking typically produces coke with a low volatiles content.
Two delayed coking methods for the production of premium and regular grade coke are common in the art. The first operates coke drums on a blocked-out (alternate) basis and uses a single fractionator for both feeds. The second method utilizes separate fractionators and related equipment for each coker system, premium and regular.
A common practice is to bring fresh feed into the fractionator column below the trapout pan where the feed is used to cool the entire coke drum vapor stream and condense the high-boiling fractions. It is also a common practice to retain a portion of the coke drum vapors as a condensed liquid (recycle) in the bottom of the fractionator where it is admixed with the fresh feed. These recycled coke drum vapors are at an appreciably higher temperature than the fresh feed and serve to preheat the fresh feed before it is sent to the furnace.
Regular grade operations are normally at low recycle ratios. Heat and material balance considerations in the fractionator result in a bottoms stream which is a combination of fresh feed and the condensed high-boiling materials from the drum overhead vapors. Recycle is used primarily to adjust the total feed carbon residue in regular grade coking operations.
Premium coking operations utilize a moderate to high ratio of recycle (condensed coke drum vapors) to fresh feed in the charge to the coke drums. The high recycle content in premium coker feed serves two functions: (1) providing a vaporizable material to generate satisfactory velocities in furnace tubes and transfer lines to prevent coke deposition prior to entry into the drum and, (2), serving as an energy carrier to the coke drums to minimize the difference between the coke drum temperature and furnace outlet temperature.